Screen runs it under a terminal, while -nc actually backgrounds it as a daemon (just like apachemysql sshd etc). Both'll get it running but I'd prefer it as a daemon personally.
The -nc option lets you start it from an init.d (there are examples in the git repository) script - so you can have it start on startup and have a tool like monit monitor and restart it if it crashes for any reason.
There's no reason you'd need to run it under screen (or its alternatives) anyway. There are several modules for logging to a logfile (mod_logfile mod_syslog), and mod_event_socket + fs_cli (or other ESL clients) lets you connect to a running process to control or monitor it, and with the /log command view the log entries at any time. I've also seen screen sessions crash occasionally which has a chance of taking freeswitch out...
-Steve